1. Field of the Invention
The present invention relates to a nickel electrode incorporated as a positive electrode for an alkaline secondary battery such as a nickel-cadmium battery, nickel-metal hydride battery, nickel-zinc battery, and the like, and more particularly, to a nickel electrode for an alkaline secondary battery, in which the coefficient of use of nickel hydroxide, which is an active substance, is high, and the swelling due to charging/discharging is restricted, so that the cycle life is long.
2. Prior Art
Conventionally, a sintered type nickel electrode has mainly been used for an alkaline secondary battery. The nickel electrode of this type is generally manufactured as follows: First, metallic nickel powder such as carbonyl nickel powder is mixed with a thickener solution produced by dissolving carboxymethyl cellulose or methyl cellulose to prepare a slurry. After the slurry is applied to a punched a metal sheet, it is baked in a reducing atmosphere such as hydrogen gas or water gas to produce a sintered base plate. Then, after the sintered base plate is dipped in a nickel nitrate solution, it is dipped in an alkali solution such as sodium hydroxide to allow the two substances to react, by which a nickel hydroxide, which is a positive electrode active substance, is yielded in fine holes distributed in the sintered base plate, and the fine holes are filled with nickel hydroxide.
For this sintered type nickel electrode, however, if an attempt is made to increase the fill of nickel hydroxide in the fine holes to achieve high capacity, it is necessary to repeat the dipping operation in the above solutions to newly yield nickel hydroxide each time, and to accumulate it sequentially. This inevitably increases man power. Moreover, when nitric acid solution is used, nitric acid ions remains in the yielded nickel hydroxide. To remove this, a chemical conversion treatment or a cleaning treatment is needed. Further, for the sintered type nickel electrode, the occupation ratio of the sintered base plate to the whole electrode is increased to maintain the strength of the whole electrode, so that the nickel electrode of this type is disadvantageous in terms of high density filling of nickel hydroxide. This presents a problem in that there is a limit in increasing the capacity density of the obtained nickel electrode.
To overcome the above problem with the sintered type nickel electrode, a pasted nickel electrode has recently begun to be used commercially.
The pasted nickel electrode is generally manufactured as follows: First, a powder of cobalt compound such as cobalt oxide is mixed, if necessary, with a powder of nickel hydroxide. After the aforementioned thickener solution is added to the mixed powder, the whole is stirred to prepare viscous paste.
Next, after this paste is put into a porous substance of a three-dimensional network structure, such as expanded nickel or nickel felt, which is not only a collector but a carrier, a nickel electrode of a predetermined thickness is formed by succeedingly performing a drying and a rolling treatment.
For this pasted nickel electrode, the high density filling of nickel hydroxide can be performed and the manufacture is simple as compared with the above-described sintered type nickel electrode. However, the nickel electrode of this type presents another problem as described below.
In charging/discharging, the coefficient of use of nickel hydroxide (Ni(OH).sub.2) decreases, and nickel hydroxide swells, resulting in the distortion of the nickel electrode.
In general, the charging/discharging reaction of the nickel electrode takes place due to the free movement of protons (H.sup.+) in the crystal of nickel hydroxide, which is an active substance.
In charging, nickel hydroxide is oxidized electrochemically and converted into .beta. type nickel oxyhydroxide (.alpha.-NiOOH). Some of nickel hydroxide turns into a .gamma. type nickel oxyhydroxide (.gamma.-NiOOH). Among these two types of nickel oxyhydroxide, .beta.-NiOOH is reduced in discharging, and returns again to nickel hydroxide. That is to say, .beta.-NiOOH takes place an oxidizing/reducing reaction reversibly in the process of charging and discharging.
On the other hand, .gamma.-NiOOH is a hexagonal crystal having a long c axis and low density as compared with .beta.-NiOOH. Therefore, the fact that .gamma.-NiOOH NiOOH is yielded in charging means that the active substance filling the porous substance swells as a whole, As a result, the distortion of the whole nickel electrode occurs. Also, in swelling, the nickel hydroxide absorbs the electrolyte, so that the proper electrolyte distribution, which has been kept between the nickel electrode, a separator, and a negative electrode is broken, causing the internal resistance of battery the to increase, by which the cycle life is shortened.
In particular, for the pasted nickel electrode, since the filling density of nickel hydroxide is high, the quantity of .gamma.-NiOOH yielded in charging increases, so that the above trouble becomes substantial.
To overcome such a problem, particularly to restrict the yield of .gamma.-NiOOH in charging, the solid solution of Cd, Zn or the like in nickel hydroxide has been proposed (refer to Unexamined Japanese Patent Publication No. S61-183868, Unexamined Japanese Patent Publication No. H2-30061, etc.)
However, in the prior art described above, the crystal structure of the nickel hydroxide changes, or the effective mass of the nickel hydroxide, which is an active substance, decreases due to the solid solution of Cd, Zn, or the like in nickel hydroxide, so that the decrease in the coefficient of use and the like are not eliminated sufficiently.
Unexamined Japanese Patent Publication No. H4-328257 discloses a nickel electrode in which nickel hydroxide showing a half breadth value of 0.8.degree./2.theta. or more in the profile of X-ray diffraction is an active substance, and Unexamined Japanese Patent Publication No. H5-41213 discloses a pasted nickel electrode in which nickel hydroxide having a 80 to 120.ANG. size of crystalline particle in the direction perpendicular to the (101) plane in X-ray diffraction is an active substance.
However, these nickel electrodes as well do not necessarily offer satisfactory performance in terms of high coefficient of use of nickel hydroxide and restriction of swelling.